The invention relates to methods and apparatus for radioactive isotope identification, and in particular to identifying radioactive isotopes from their gamma-ray emission spectra.
Different risks are associated with different radioactive sources. Because of this it is often important to be able to identify the nature of a source of gamma-rays (i.e. which radioactive isotope(s) which are present). Device for doing this are generally known as radioactive-isotope identifying devices (RIDs). RIDs may be hand-held scanning devices, or larger fixed devices, such as portal monitors for people or vehicles to pass through.
RIDs are useful in a number of situations. For example, they can be used to monitor legitimate uses of radioactive material and to maintain an awareness of isotopes present in an environment where radioactive material is used. This can be important, for example, in medical and industrial applications. RIDs can also be used as general scanning devices to identify “orphaned” or other unknown radioactive sources in an environment. Furthermore, an ever increasingly important area of use for RIDs is in policing the illegitimate trafficking of nuclear materials.
Policing illegitimate trafficking of nuclear materials is one of the most challenging areas for isotope identification. This is because it often requires large numbers of people and cargos to be scanned quickly (for example as they pass through a facility such as a port, or other border crossing, at normal speed), but with a high degree of reliably. Not only is there the clear desire to be able to reliably correctly identify undesirable radioactive isotopes, e.g. plutonium, passing through a facility, it is also important that legitimate sources of radiation are not wrongly identified as undesirable sources. This is because false-alarms arising from the kind of mis-identification at border crossings, for example, can be very costly, both financially in terms of lost operation time while the alarm is investigated, and in terms of the degree of inconvenience to people passing through the facility.
However, it can be difficult to avoid false-alarms because many legitimate sources of naturally occurring gamma-ray radiation may be present where undesirable sources of radiation are being sought. For example, sanitary ware, roofing tiles, cat litter and scouring pads, which are all common sources of naturally occurring radiation, frequently pass through border crossings in the course of normal trade. What is more, some legitimate sources of radiation have features in their gamma-ray emission spectra which are similar to those in some undesirable sources. For example, radioactive isotopes of iodine (for medical use) and barium (for industrial use) both emit gamma-rays at energies which are similar to gamma-rays from weapons grade plutonium. Not only does this mean that innocent cargos may give rise to false-alarms, it also means that those involved in illegitimate trafficking of nuclear materials could seek to mask weapons grade plutonium by hiding it among a cargo of medical iodine, for example. Because of this, there is a desire for RIDs to be as accurate and reliable as possible.
RIDs typically comprise a gamma-ray spectrometer component and a processor component. The gamma-ray spectrometer component is for obtaining gamma-ray emission spectra from objects under investigation. The processor component is for determining the most likely source of the gamma-rays (i.e. the radioactive isotopes present) on the basis of characteristics of the spectra obtained.
One class of RID currently in use is based on high-purity germanium (HPGe) detectors, for example the Detective-EX-100® RID from the ORTEC Corporation. Another class is based on room temperature semiconductor detectors, such as cadmium zinc telluride (CdZnTe or CZT), for example the Interceptor™ RID from the Thermo Electron Corporation. Both types of spectrometer are able to provide high-resolution gamma-ray spectra (which is key to reliable isotope identification). Another class of RID comprises those based on scintillation spectrometers.
Each of these classes of RID have advantages and drawbacks. For example, HPGe and CZT detectors are able to provide high resolution spectra which allows similar source spectra to be distinguished. However, HPGe and CZT detectors are also typically small volume devices. This makes them difficult to implement in large RIDs of the kind required by some applications, e.g. for quickly scanning lorries passing through a port. HPGe detectors also require cooling. This adds to their weight, cost and complexity, and can make it difficult to implement and support them in portable hand-held applications. Scintillation-body based spectrometers, on the other hand, are relatively cheap and easy to implement on both small (e.g. portable hand-held RIDs) and large (e.g. vehicle portal monitor RIDs) scales, but have relatively low intrinsic spectral resolution and relatively low signal-to-noise ratios. Because of this, HPGe and CZT-based spectrometers have generally been seen as preferable to scintillation-body based spectrometers for use in RIDs.
Once a gamma-ray spectrum has been obtained using the spectrometer component of the RID, the spectrum is analysed by the processor component of the RID to identify what isotope(s) are the most likely source(s) of the observed spectrum. To some extent this process is independent of which class of spectrometer is employed in the spectrometer component of the RID.
One way of analysing the measured gamma-ray spectrum is by a full spectrum analysis. According to this approach, the processor component of the RID cycles through a collection of “templates” (i.e. pre-computed gamma-ray signatures for nuclear materials expected to be encountered) and compares them with the observed spectrum, for example using a correlation function. Identification is made by choosing the template, or linear combination of templates, which best matches the observed spectrum.
There are, however, a number of drawbacks with this approach. Firstly, it is computationally intensive. This can be especially problematic in portable hand-held devices where processing capability is more restricted than in larger fixed installations. Secondly, the templates can only accurately reflect the gamma-ray signatures from particular isotopes under given laboratory conditions. The gamma-ray signature from a source “in the real world” is likely to be different to that seen in the laboratory. This is because the recorded spectrum will be affected by scattering and absorption within materials surrounding the source, and also will be affected by the specific geometry of the source being investigated, and its orientation with respect to the RID. These effects are generally energy dependent. This means the recorded spectrum for a given isotope will not normally properly match the stored laboratory-conditions template for that isotope. For example, the template for a given isotope may include two emission lines of similar intensity, one at a relatively low energy, and one at a relatively high energy. If this isotope is in a person's hand luggage, it is likely that both lines would be seen with similar intensities, and the isotope could properly be identified from this. However, if the isotope is carried through a portal monitor hidden in a cargo of concrete, the low energy line would be adsorbed more than the high energy line and the recorded spectrum would not match the template. This makes it more difficult to properly identify the isotope and so can lead to mis-identification. For example, it means relatively loose constraints must be used for making a positive identification of an isotope (e.g. a relatively low correlation coefficient between the recorded spectrum and the template). This leads to increased likelihood of false-positive alarms. Accordingly, this kind of full spectrum analysis is inflexible because it cannot take proper account of how the gamma-ray signatures from a source are modified by the environment in which they are measured.
An alternative approach for identifying isotopes from a gamma-ray spectrum is the so-called single multi-component analysis method [1]. In this method, the starting point is again the computation of a range of template spectra for isotopes expected to be encountered in the application at hand. The most likely source composition is then determined from a principle component analysis of the recorded gamma-ray emission spectrum from the source. Multiple templates are provided for each potential radio-nuclide. The multiple templates correspond to the different predicted signatures from each isotope for a range of different shielding materials and geometries. For example, one template might correspond to unshielded plutonium, while another might correspond to plutonium shielded by 2 metres of concrete. This approach can help to address some of the problems which arise from the same radioactive source isotope giving rise to different recorded gamma-ray spectra in different situations.
However, relying on multiple templates for each isotope in this way increases further still the computational power required to implement the method in a reasonable time (for example in the time taken for a person to walk past a detector). Furthermore, the method still necessarily employs a finite number of templates to model what is in effect an infinite number of different shielding and geometry arrangements. This means it is still not possible to fully account for all of the different ways in which a recorded gamma-ray spectrum can be affected by the environment. Accordingly, ambiguities in isotope identification can still arise.
There is therefore a need for a method of identifying isotopes which is less computationally intensive than known methods, and which is also less prone to false identifications (false alarms) caused by environmental effects modifying the observed spectra so that isotope identification can be made with a greater degree of confidence.